The present invention relates to a silicon layer which is highly sensitive to oxygen as well as to a method for obtaining this layer.
It is notably applied to microelectronics.
Silicon carbide (SiC) is a very interesting compound semiconducting material IVxe2x80x94IV, which is particularly suitable for high power, high voltage or high temperature devices and sensors.
Recently, very significant advances have been accomplished in the understanding of the surfaces of this material and of the SiC interfaces with insulators and metals.
Two of the important issues for the success of SiC-based electronic devices (and particularly those which are based on hexagonal polytypes of this material) relate to obtaining high performance MOS transistors, surface passivation and therefore oxidization of SiC, and the insulating structure on SiC.
Let us note that silicon is presently the most used semiconducting material, mainly because of the exceptional properties of silicon dioxide (SiO2).
From this point of view, SiC is of particular interest as its surface passivation may be achieved by growing SiO2 under conditions similar to those for silicon.
However, because of the presence of carbon, conventional oxidization (direct SiC oxidization) of the SiC surfaces (particularly of the hexagonal surfaces of this material) generally leads to the formation of Si and C oxides, which have poor electrical properties, and to SiO2/SiC interfaces which are not sharp, the transition between SiC and SiO2 occurring over several atomic layers.
Electron mobility in the MOS structure inverting layers on p-SiC is much lower (by a factor 10) than on silicon, because of disorder at the interface.
The object of the present invention is to find a remedy to the above drawbacks.
Its object is a silicon layer which favors the growth of an oxide on a substrate considerably and leads to a SiO2/substrate interface which is sharp, the transition between the substrate and SiO2 virtually occurring over a few atomic layers.
Specifically, the object of the present invention is a silicon layer formed on a substrate, this layer being characterized in that it has a 4xc3x973 surface structure (it is also said to be 4xc3x973 reconstructed), the substrate being able to receive this 4xc3x973 surface structure of silicon or suitable for favoring the formation of this structure.
Preferably, the substrate is made out of a material selected from silicon carbides and silicon.
The silicon carbide may be monocrystalline (cubic, hexagonal (more than 170 polytypes) or rhombohedral form), polycrystalline, amorphous, or porous.
As an example, the layer is formed on a 6Hxe2x80x94SiC (0001) surface, a reconstructed 3xc3x973xc3x973, 63xc3x9763 or 1xc3x971 surface for example.
The present invention also relates to a method for obtaining the layer, object of the invention.
According to this method, silicon is deposited on a surface of the substrate in a substantially uniform way.
According to a preferred embodiment of the method, object of the invention, a surface of the substrate is prepared for receiving the silicon layer, the substrate is heated to a high temperature, at least 1000xc2x0 C., silicon is deposited onto the surface of the thereby heated substrate in a substantially uniform way, at least one annealing of the substrate is performed, whereon silicon is deposited at least at 1000xc2x0 C., the total annealing time being of at least 5 minutes, and the substrate is cooled at a rate of at least 100xc2x0 C./minute.
Preferably, when the substrate is made out of a monocrystalline silicon carbide, silicon is deposited on this substrate heated to about 650xc2x0 C., the substrate is then annealed at 650xc2x0 C. at the least, the total annealing time being of at least 7 minutes, and then cooled at a rate of at least 50xc2x0 C./minute.
Preferably, particularly when the substrate is made out of monocrystalline silicon carbide, the preparation of the surface of the substrate for receiving the monocrystalline silicon and/or for promoting its formation, comprises auxiliary heating of the substrate to at least 1000xc2x0 C., auxiliary substantially uniform deposition of monocrystalline silicon on the surface of the thereby heated substrate and at least one auxiliary annealing of the substrate after this auxiliary deposition, at 650xc2x0 C. at the least, with the total auxiliary annealing time being of at least 7 minutes.
Before the auxiliary heating, the preparation of the surface of the substrate preferably comprises degassing of the substrate under ultra-high vacuum and then at least one annealing of this substrate and then a cooling of the substrate.
In the present invention, silicon is preferably deposited by vacuum evaporation.
According to a preferred embodiment of the invention, silicon is deposited from a surface of a silicon sample, this surface of the sample being larger than the surface of the substrate.
Preferably, the surface of the silicon sample and the surface of the substrate are separated by a distance of the order of 2 to 3 cm.
The present invention also relates to a method for obtaining a silicon dioxide layer, characterized in that a silicon layer is made on a substrate in accordance with the method for obtaining a silicon layer, object of the invention, and this silicon layer is oxidized.